Conventional communications systems include a receiver for receiving and processing transmitted waveforms. The receiver may have a tuner that receives a modulated waveform with a particular frequency bandwidth. The tuner provides analog gain and frequency translation functions necessary for converting the input signal to an intermediate frequency (IF). The IF signal is then shifted down to baseband frequency (i.e., f=0Hz) and separated into its in-phase component (I) and its quadrature-phase component (Q).
DC offsets at baseband signal stages are unavoidable due to the inherent inaccuracies of analog signal processing and analog-to-digital converters. DC offset is a serious problem in digital demodulators, especially those employing high dc-gain elements such as decimators. DC offset also has a detrimental impact on automatic gain control (AGC), carrier recovery and hard and soft decision circuits.
A conventional demodulator circuit receives an input signal at intermediate frequency and converts that signal to a baseband signal. Such a conventional circuit includes a multiplier, a synthesizer, an analog-to-digital converter and a filter. The synthesizer generates a frequency equal to the frequency f.sub.IN of the input signal S.sub.IN. The multiplier multiplies the input signal S.sub.IN with the output of the synthesizer to convert the input signal S.sub.IN to baseband frequency (i.e., shifted in frequency to 0Hz). The input signal is then digitized by an analog-to-digital converter and filtered through a low pass filter. However, when the baseband signal is filtered through a low pass filter. However, when the base band signal is filtered through a low pass filter, both the base band signal of interest and the undesirable DC offset are output. Thus, the DC offset is present in the output where it will have a detrimental effect on the remainder of the circuitry in which the baseband signal is further processed. Such a result is undesirable.
Conventional DC offset correction involves the measurement of DC offset and subtraction of the measured DC offset from the signal of interest. Such a method is computationally intensive and inaccurate especially in the presence of low frequency baseband signals. In addition, the DC offset measurement is sensitive to DC drift over time and temperature changes especially when the DC offset measurement is performed beforehand when zero signal conditions may exist.
It is thus desirable to eliminate or at least minimize DC offset in a simple and reliable manner. In particular, it is desirable to eliminate the need for a measurement or correction phase which is sensitive to temperature and time drift.